1. Field of the Invention
The present invention relates to control devices and control methods used in various fields, such as temperature control. More particularly, the present invention relates to a technique of generating a setpoint path and suppressing overshoot due to control in response to application of a disturbance or a change in a setpoint.
2. Description of the Related Art
Techniques of the related art for suppressing overshoot due to control in response to application of a disturbance or a change in a setpoint include a technique disclosed in Japanese Patent No. 4310804. According to the technique disclosed in Japanese Patent No. 4310804, a generation setpoint path SLSP is given to a proportional-integral-differential (PID) control computation unit so that a process variable PV settles on a user-specified target setpoint SP in accordance with a characteristic curve that represents a characteristic of the process variable PV on the phase plane. Specifically, the generation setpoint path SLSP is calculated as SLSP=SP+ΔPV/k using a rate ΔPV of change of a deviation (SP−PV) or of the process variable PV, and a gradient k of the characteristic curve.
Another technique for suppressing overshoot involves a setpoint ramping function included in ordinary controllers. The setpoint ramping function is a function of generating a linear (ramping) setpoint path by allowing a user to specify a period taken by a generation setpoint path given to the PID control computation unit to reach the target setpoint when the user changes the target setpoint. Since the generation setpoint input to the PID control computation unit ramps and the change in the generation setpoint becomes gradual, the setpoint ramping function is effective for suppressing overshoot when the target setpoint is changed.
In the technique disclosed in Japanese Patent No. 4310804, the setpoint path is generated using the rate of change of the deviation or of the process variable in every control computation cycle. Since the technique uses the process variable containing uncertainties (such as dynamic characteristics of the control target, measurement noise, and environmental factors) to generate the setpoint path, it is practically impossible to specify in advance a period for the generation setpoint to reach the target setpoint.
Accordingly, if the technique disclosed in Japanese Patent No. 4310804 is applied to a control target for which the target setpoint is repeatedly changed or a disturbance is repeatedly applied at a predetermined interval, the next target setpoint change or the next disturbance application may occur before the generation setpoint reaches the target setpoint. Examples of the circumstance where the target setpoint is repeatedly changed include a circumstance where the target setpoint (temperature setpoint) is repeatedly changed in a chemical manufacturing apparatus. Examples of the circumstance where a disturbance is repeatedly applied include a circumstance where temperature alters because printed circuit boards subjected to soldering are periodically put into a reflow oven having a constant target setpoint (temperature setpoint).
FIGS. 7A and 7B are diagrams for describing technical issues of the technique disclosed in Japanese Patent No. 4310804. FIG. 7A is a diagram illustrating an example of a control response when the target setpoint is changed, whereas FIG. 7B is a diagram illustrating an example of a control response when a disturbance is applied. In FIGS. 7A and 7B, SP+ denotes a target setpoint specified by a user, PV denotes a process variable, and ΔSP denotes a shaping amount of the generation setpoint relative to the target setpoint SP+ (difference between the target setpoint SP+ and the generation setpoint).
A simulation result illustrated in FIG. 7A indicates that, with the technique of the related art disclosed in Japanese Patent No. 4310804, a desirable control response for which overshoot of the process variable PV is suppressed is obtained for the first change in the target setpoint SP+ but the reproducibility decreases as the change in the target setpoint SP+ is repeated. In addition, a simulation result illustrated in FIG. 7B indicates that a control response for which overshoot is suppressed is obtained for the first application of a disturbance but the reproducibility decreases as application of a disturbance is repeated.
Although not explicitly described in Japanese Patent No. 4310804, examples of a conceivable process used to address the issues described above include
(I) a process of continuously deriving a generation setpoint path (hereinafter, referred to as a following generation setpoint path) from the final value for a generation setpoint (hereinafter, referred to as a preceding generation setpoint) along a generation setpoint path immediately preceding the following generation setpoint path, and
(II) a process of performing some kind of initialization when the following generation setpoint path is started to make the preceding generation setpoint less likely to affect the following generation setpoint path.
In the case of (I), the preceding generation setpoint affects the following generation setpoint path, and the reproducibility of a control response against every change in the target setpoint and every application of a disturbance greatly decreases.
In the case of (II), since initialization of the generation setpoint causes discontinuity in the deviation, a discontinuous action also occurs during control using this deviation and disturbs the control response. Since how the control response is disturbed changes depending on the final value of the preceding generation setpoint, the value of the generation setpoint path after a specified period has passed is not guaranteed. Consequently, the reproducibility of control is not realized every time.
In addition, when a setpoint given to the PID control computation unit is generated using the setpoint ramping function included in ordinary controllers, the change in the generation setpoint takes a discontinuous behavior in which the change abruptly becomes zero upon the end of ramping. If this configuration is applied to controllers having derivative compensation, such a discontinuous behavior disturbs a control response and causes a control output to decrease upon the end of ramping, and consequently settling of the process variable takes longer. In addition, since the setpoint ramping function is for changing the target setpoint, it fails to suppress overshoot when a disturbance is applied.